Glacier Peak’s lahars once left 7 feet of sediment at Arlington, 60 miles downstream

Seven feet of volcanic mud and sediment sit in the ground at Arlington, Washington, laid down roughly 13,100 years ago by lahars that rushed off Glacier Peak and traveled more than 60 miles. The U.S. Geological Survey cites that deposit in its lahar hazard assessment for the valleys below the peak, and it is the figure that most plainly shows how far a Glacier Peak mudflow can travel.

Arlington sits in the Stillaguamish River valley, a different drainage from the ones that carry most of the volcano’s runoff. That detail is the most interesting part of the record.

Arlington’s 7-foot lahar deposit, 60 miles from the crater

The wording comes from the USGS Volcano Hazards Program. On its Glacier Peak lahar hazards page, the agency states: “In the Stillaguamish River valley at Arlington, more than 95 km (60 mi) downstream from Glacier Peak, lahars deposited more than 2 m (7 ft) of sediment.” Both numbers carry the qualifier “more than,” so the figures are minimums for that layer rather than a measured average.

USGS Fact Sheet 058-00, “Reducing Risk from Volcano Hazards: Glacier Peak, History and Hazards of a Cascade Volcano,” dates the event. About 13,100 years ago, during the earliest of the major eruptive episodes in the record, lahars flowed down both the North Fork Stillaguamish and the Skagit Rivers all the way to the sea, and “lahars deposited more than seven feet of sediment” at Arlington.

The dated eruptive record, 13,100 years ago to 300 years ago

The Arlington layer is the oldest of a series. The same fact sheet lists major eruptive episodes about 13,100, 5,900, 1,800, 1,100 and 300 years ago. About 5,900 and 1,800 years ago, dome-building eruptions sent lahars to the sea again, this time only down the Skagit. USGS’s profile of the volcano puts the most recent eruption at roughly 1,100 years ago and rates its threat potential “Very High” under the National Volcano Early Warning System, which scores a volcano on its hazards and the exposure of people and property.

The record was first assembled from field exposures by geologist J. E. Beget, whose 1982 USGS Open-File Report 82-830 mapped postglacial deposits on the volcano’s flanks. It traced an early clayey mudflow at least 35 km down the White Chuck River valley and lahar deposits preserved as much as 100 km downstream.

Later work by L. G. Mastin and R. B. Waitt of the Cascades Volcano Observatory, in a USGS report on Glacier Peak’s eruptive history, divided the last 14,000 years into two major episodes, one from 13,100 to 12,500 years ago and another from 6,300 to 5,900 years ago. In their assessment, lahars from the earlier episode ran all the way to Puget Sound and delivered the Arlington sediment, while smaller lahars of the past 5,900 years reached only as far as the mouth of the White Chuck River.

Distance is part of why the site matters. The USGS profile places Glacier Peak in Snohomish County, about 65 miles northeast of Seattle, at 10,541 feet, in the Glacier Peak Wilderness Area of Mt. Baker-Snoqualmie National Forest. Few people live near the summit. Arlington lies more than 60 miles downstream in a separate river valley, so a thick deposit there says far more about reach than any layer found near the crater would.

The Sauk detour to the Stillaguamish

Arlington’s place in the record raises an obvious question, since the Stillaguamish is not Glacier Peak’s own river. The lahar hazards page answers it directly: lahars in that valley are the least likely of those mapped, and would occur “only if the Sauk River became choked with enough debris to be diverted west into the Stillaguamish River valley.” The 13,100-year-old deposit shows that lahars have reached that valley at least once, and at a thickness of more than seven feet.

For the rest of the system, USGS’s hazards overview adds that the volcano “tends to erupt explosively,” with the most serious hazards reaching population centers relatively far from the peak, and the lahar page describes lahar effects as most frequent in the White Chuck and upper Suiattle valleys and less frequent but potentially more damaging in the Sauk and Skagit valleys. A lahar there could, in the agency’s words, “block transportation routes, destroy highways and bridges, bury houses in mud, cover farmland with debris, choke river channels, and increase the severity of floods for years or decades after the eruptions stop.”

The deposit also says something about odds. Mastin and Waitt put the chance of a major eruption in a human lifetime as “fairly low, perhaps one in a few hundred,” even though the 14,000-year record shows at least a dozen eruptions. A 7-foot layer at Arlington is a statement about reach, not about timing: it shows how far the largest known lahars traveled, and USGS gives no schedule for the next one.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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